"Free wheel" or “one way” gear. Spins in one way but not in the other. I made it for a 8x16x5(it's less than a dollar on aliexpress, even the high quality ones) bearing to be placed in between the 2 big parts. You need 4 of the small part, and optionally a small pen spring to push them outwards.  P.S. My printer died so I didn't get a chance to test it, let me know how it works.